Method and composition for controlling corneal hydration

ABSTRACT

An ophthalmic solution for treating corneal edema concurrent with epithelial discontinuity, healing epithelial defects, and protecting against epithelial trauma containing electrolytes at only isotonic levels but containing hydrophilic macromolecules at such concentration that their collodial osmotic pressure is equal or greater than the imbibition pressure of corneal stroma.

This is a continuation of application Ser. No. 227,819, filed Jan. 23,1981, now abandoned, which is a division of Ser. No. 06/046,188, filed6-7-79, now U.S. Pat. No. 4,271,144.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a multifunctional aqueous ophthalmicpreparation that is designed to be used topically in the eyes of humansand domestic animals. The present invention further relates to theprovision of an aqueous solution of physiological pH, havinghyperosmolality with respect to corneal stroma of the colloidalcomponents only, which is useful for dehydrating edematous cornea evenin the presence of damaged corneal epithelium when the barrierproperties to small molecular weight electrolytes of the epitheliallayer is lost. The invention also relates to an ophthalmic agent that isbeneficial in promoting the permanent healing of epithelial defects.Still further, the invention relates to a solution that is capable ofprotecting the corneal epithelium from injury such as abrasion that mayresult from close and prolonged contact with solid surfaces, such as inthe diagnostic procedures of tonography and tonometry, or any otherprocedure where a foreign solid object is brought in close contact withthe preocular surface. Furthermore, the invention relates to anophthalmic solution that can be used for irrigating purposes duringprolonged intraocular surgical operations, e.g. vitrectomy, to preservecorneal thickness and clarity and to protect the cellular interfaces sothat the incidence and the severity of postoperative complicationsinvolving the corneal epithelium or endothelium can be prevented. Theinvention also relates to the attainment of all the foregoing withsignificant decrease of ocular discomfort if present, and withoutoptical interference with visual acuity. The invention further relatesto an ophthalmic solution that can be formulated to have bactericidalactivity without detectable decrease in its therapeutic efficacy.

2. Description of the Prior Art

The most powerful refractive medium of the eye, the cornea has to betransparent and must have an optically smooth surface to fulfill itsfunction, the formation of a sharp visual image on the retina. Thecornea is a normally transparent, mostly acellular connective tissueconsisting of collagen fibers and mucopolysaccharides. This tissue iscalled the stroma, which is covered by several layers of epithelialcells on the exterior surface, while its interior surface is coveredwith a single layer of endothelial cells. The macromolecules of thestroma form a loosely connected matrix that contains about 80% by weightof aqueous fluid. This interstitial fluid of the stroma is nearlyisotonic. The tissue, however, behaves as if it were somewhatdehydrated, since it tends to imbibe, i.e. absorb, additional fluid whenimmersed in physiological saline. This occurs because the additionalosmolality of its macromolecular matrix causes a net flow of water intothe stroma by osmosis, with the epithelium acting as a semipermeablemembrane. Osmolality is a property of a solution which depends on theconcentration of the solute molecules per unit weight of solvent anddetermines the amount of solvent absorbed by a particular substrate,usually in the presence of a semipermeable membrane. This tendency ofwater absorption is measured in terms of the so-called "imbibitionpressure", which is about 40-60 torrs for the normal cornea. As thedegree of hydration of the stroma increases, the imbibition pressurediminishes. Active transport of electrolytes and thus water out of thestroma by its boundary layers of cells keeps the stromal hydration atits normal, i.e. somewhat dehydrated, level in order to maintain itstransparency. As the cornea imbibes water, it becomes progressively morecloudy diminishing visual acuity. A highly edematous cornea scatters somuch light that it appears to be quite opaque.

The superficial epithelium can also become edematous. In such a case,droplets of fluid accumulate in the dilated intercellular spaces of theepithelium. Edematous epithelial tissue can loose its integrity; thecellular adhesiveness is lowered to such a degree that it can easily beabraded.

Corneal edema can result from various causes. A malfunction of thecorneal endothelium usually results in severe corneal edema. Traumatizedcorneal epithelium, recurrent epithelial erosion, and epithelialdiscontinuities of other types can also result in corneal edemadrastically reducing visual acuity.

Heretofore, corneal edema of sufficient severity to result in blurredvision has been treated with hypertonic salt solutions i.e. solutionswhich cause water to flow out of the cornea by having a higher tonicitythan the solution within the cornea. Aqueous solutions of sodiumchloride at concentrations of 2% and 5% by weight, instilled severaltimes a day may provide symptomatic relief and thus temporarily restorevisual acuity. Hydrogel lenses have also been used in conjunctin withhypertonic salt solutions and have been able to prolong the dehydratingeffect of the hypertonic salt solution somewhat.

In addition to hypertonic sodium chloride solutions, hyperosmoticointments containing 40% glucose are also employed in clinical practice,but its dehydrating effect is only slightly higher than that of the 5%sodium chloride solution. Hyperosmotic compositions are those containinga higher concentration of osmotically active components than thatcontained within a physiological salt solution containing 0.9% by weightof sodium chloride. Anhydrous glycerol is a highly effective dehydratingagent, but it is poorly tolerated by the eye of most patients asindicated by considerable discomfort, conjunctival injection, andphotophobia, i.e. an abnormal visual intolerance of light.

All the foregoing hyperosmotic preparations have one weakness in common.They only have the desired dehydrating effect on the cornea, if theepithelium, which acts as an imperfect semipermeable membrane, isintact, since osmotic effects can occur only across a membrane thatexcludes at least one of the solutes. Hence, in many patients havingcorneal edema which is associated with epithelium of poor integrity,i.e. having epithelial discontinuities, the symptomatic treatment withhypertonic salt solutions or glucose ointment is ineffective, and mayeven be harmful, as the solute can readily penetrate the stromaachieving hypertonic levels inside the tissue which causes furtherimbibition of water with the resulting clouding of the cornea whichdiminishes visual acuity. Even in the presence of intact epithelium, theeffect of hypertonic salt solutions is short-lived, as the drops arediluted and the excess salt is washed out of the eye by reflex tearing.

Some attempts have been made to improve the observed poor performance ofhypertonic salt solutions especially in cases of corneal edemaconcurrent with epithelial discontinuity. Artificial tears containing"mucomimetic" polymers, i.e. polymers which mimic the action ofnaturally-occurring mucin which acts as a wetting agent in the eye andthus is responsible for tear film stability, have been used asophthalmic vehicles for sodium chloride at hypertonic levels. However,in these polymeric solutions the osmolality of the dissolved polymers isnegligibly small, and thus these attempts have added little to theperformance of the hypertonic solutions other than lessening thediscomfort upon application.

It is accordingly an object of the present invention to includecolloidal components in the ophthalmic formulations at sufficiently highconcentration so that their osmotic pressure is equivalent to orsupercedes the imbibition pressure of a deturgescent cornea. Thecolloidal components have to be of such molecular size and shape thatthey cannot penetrate readily even the denuded stroma. The colloidalsubstance used also has to be sufficiently water-soluble and of lowviscosity so that the required high solution concentration can beachieved before the limit of solubility is reached and without the highconcentrations resulting in impractically high viscosity. A furtherobject of the present invention is the provision of such solution withcolloidal hyperosmolality where the hydrophilic polymers includedreadily adsorb at the cellular and tissue interfaces thereby prolongingthe beneficial osmotic effect. It is also an object of this inventionthat the polymer layer adsorbed at the bio-interfaces should exert abeneficial effect on the healing of epithelium, on the adhesion ofregenerating epithelium to the stroma and provide protection for thecorneal epithelium against mechanical abrasion and other trauma besidesprolonging the dehydrating osmotic effect for edematous corneas.

SUMMARY OF THE INVENTION

These and still other objectives, as will become apparent from thefollowing disclosure, are attained by the composition of the presentinvention, which in its broadest term comprises a hydrophilic, highmolecular weight polymer dissolved in isotonic aqueous electrolytesolution at sufficiently high concentration to effect a colloidalosmotic pressure equal to or greater than that of a deturgescent cornea,i.e. between about 40 and about 200 millimeters of mercury. Highlywater-soluble polysaccharides or polyamides with an average molecularweight ranging from about 20,000 to about 150,000 daltons are preferredthat can effect the osmotic pressure desired at solution concentrationsranging from 5% to 25%, that have the hydrophilicity, adsorptivity, andlow viscosity even at high solution concentrations that are required toachieve the desirable effects. These substances are also nontoxic andare highly compatible with the macromolecular tear components, theintercellular adhesive of the epithelium, and the glycosaminoglycansthat form the ground substance of the stroma.

DETAILED DESCRIPTION OF THE INVENTION

A preferred colloidal component for use in the ophthalmic solution ofthe present invention is dextran, a polymer of glucose that is chieflyjoined through α-1,6-glycosidic linkages, and is obtained by appropriateprocessing of the high molecular weight product derived from thefermentation of sucrose by the bacteria Leuconostoc mesenteroides. Theophthalmic solution component has an average molecular weight of atleast about 40,000 and not more than about 150,000 daltons, and isobtained by partial hydrolysis and fractionation of the high molecularweight fermentation product. This polysaccharide is highly water-solubleforming aqueous solutions of low viscosity, it carries no net electriccharge, and its effect on water surface tension is negligible. Anotherpreferred hydrophilic polymer useful as the colloidal component of theophthalmic solution of the present invention is the synthetic polymerpoly (acrylamide) having an average molecular weight between about20,000 and about 150,000 daltons. This polymer is also highlywater-soluble forming aqueous solutions of low viscosity, the moleculecarries no net electric charge and it has a negligible effect on watersurface tension. The respective techniques of producing these polymersare well established and form no part of the present invention.

Various molecular weight dextrans are described in U.S. Pat. No.4,039,662 as being useful in preparing artificial tear solutions whenused in combination with benzalkonium chloride. U.S. Pat. No. 3,920,810describes polyacrylamide containing ophthalmic solutions as artificialtear solutions. In addition to using these ophthalmic solutions asartificial tear materials, they are also suggested as being useful aslubricating and cushioning agents for the eye after an injury orsurgery, or as cleaning, lubricating, and cushioning agent for hard andgel type contact lens. However, neither patent teaches nor suggests thedesirability of preparing a solution of such a polymer at aconcentration sufficient to produce a solution having a colloidalosmolality equal to or exceeding the collodial osmolality of adeturgescent cornea. Furthermore, neither patent teaches nor suggeststhe beneficial effects resulting from controlling corneal hydration bythe use of such a solution in the eye as described herein. The factorsinfluencing corneal hydraton are discussed in InvestigativeOphthalmology, Vol. 1, No. 2, pp 151-157 (1962) by J. E. Harris. Whilethat article discusses corneal hydration in general and the desirabilityof dehydrating the cornea in certain instances, only two materials arediscussed for use in the eye. The first was a 25% solution of scrumalbumin which had the disadvantages of being somewhat toxic to theepithelium, as well as being rapidly lost from the conjunctival sac. Thesecond was a 5 % solution of a low viscosity carboxymethylcellulosesodium salt having a molecular weight of about 40,000, which was said tohave some merit in cases where the stroma could be acted on directly butwas of no value when the epithelial tissue was still partially intact,e.g. bulious keratopathy or epithelial edema, probably due to its ionicnature, sodium ion content, the comparatively high viscosity ofcarboxymethylcellulose as opposed to the polymers of the presentinvention, and the relatively low concentration of polymer used. Whiledextran is mentioned therein as having been used in vitro on excisedpieces of cornea, there is no indication of its use in the eye nor ofthe exact molecular weight of the polymer which was used.

Aqueous solutions of dextran and poly(acrylamide) are highly stable andare compatible with the biopolymers found in tears. Both polymers alsohave very low toxicity. Since these polymers are nonionic, theconcentration of inorganic salts needed to achieve physiologicaltonicity has no appreciable deleterious effect on their solubility sothat polymer solutions at concentration levels needed to achieve andeven surpass the colloidal osmolality of the deturgescent cornea canreadily be formulated even in the presence of inorganic electrolytes.

It is preferred that the ophthalmic solution of the present invention beisotonic. Any of the salts described as useful in the prior art forrendering an ophthalmic solution isotonic may be used in the presentinvention, such as, for example, sodium chloride, potassium chloride,calcium chloride, magnesium chloride, and various sulfates, phosphates,borates, nitrates, citrates, acetates, etc.

If desired, it is also possible to add a preservative to the ophthalmicsolution used in the present invention. For example, biocides such asbenzalkonium chloride, thimerosal, phenylmercuric nitrate,chlorobutanol, methyl paraben, propyl paraben, chlorhexidinedigluconate, and sorbic acid and chelating agents, such as for example,di, tri, or tetrasodium ethylene diamine tetraacetate, also known asedetates, may be added at concentrations between about 0.001% and 0.1%by weight.

In addition, the composition of the present invention can also containas an optional ingredient an eye compatible anesthetic such as, forexample, benoxinate, butyl-4-amino-benzoate, naepaine, and phenacaine. Apreferred anesthetic is proparacaine.

Furthermore, the composition of the present invention may also be usedas a carrier for ophthalmic medicants, for example: mydriatics such astropicamide, atropine, and epinephrine; miotics such as pilocarpine andcarbachol; cycloplegics such as cyclopentolate; anti-inflammatories suchas dexamethasone and prednisolone; anti-infectives such as sulfas andantibiotics; and vasoconstrictors such as phenylephrine and naphazoline.The medicants may be present in the form of their pharmaceuticallyacceptable salts or esters.

Another optional ingredient of the composition of the present inventionis an eye compatible fluorescing compound of the type used influorophotometric determinations such as that used when fitting contactlens. Examples of such fluorescing compounds include sodium fluorescein,with a preferred fluorescing compound being fluorexon.

The dehydrating solution of the present invention, a concentratedaqueous solution of certain hydrophilic colloids, is useful in a numberof contexts. Primary among these is the intensive dehydrating effect onedematous cornea even in the complete absence of epithelium, whichquickly results in improved visual acuity. The solution also exerts ahydrating effect of edematous epithelium of poor integrity removing theintercellular pools of fluid accumulated in the epithelium and therebyincreasing the cell to cell and the cell to substrate (basementmembrane) adhesion. A related effect is the lessening or even completedisapperance of the occasional severe discomfort experienced by patientswith damaged corneal epithelium.

While not wishing to be bound by theory, it is believed that thebeneficial effect of the macromolecular dehydrating component containedin the ophthamlic solution of this invention appears to be manifested byboth bulk and interfacial means. There are indications that despitetheir low surface activity and lack of electric charge, certainhydrophilic polymers, such as the ones described herein, adsorb at thecellular and tissue boundaries and the observed beneficial effect onstromal hydratic epithelial healing, and epithelial adhesion is at leastpartly due to the presence of this adsorbed macromolecular layer whichforms an osmotic gradient at the cornea-tear interface. Otherhydrophilic polymers, although similar in molecular weight andsolubility, fail to produce the desired effect in vivo, and it isbelieved that such failure is due to the failure of such solutions toform an osmotic gradient at the cornea-tear interface.

The attainment of the objectives of formulating an ophthalmic solutionthat can be used to manage corneal edema in the presence of injured orabsent epithelium and thereby restore and preserve visual acuity, toprotect corneal epithelium from injury resulting from mechanical orchemical trauma or manipulation during surgical interventions, and topromote the proliferation and attachment of corneal epithelium therebyavoiding subsequent complications, is illustrated by the followingexamples, which are intended to be purely exemplary of the use of theinvention.

EXAMPLE I

Dextran-40 polymer having an average molecular weight of about 40,000daltons and sodium choride are dissolved in distilled water in thefollowing proportions:

    ______________________________________                                        Dextran-40          6.00   grams                                              Sodium chloride     0.90   grams                                              Distilled water     93.1   milliliters                                        ______________________________________                                    

The colloidal osmolality of such a solution is about 3 mOsm, whichcorresponds to an osmotic pressure of 57 mm Hg. The solution can bereadily heat-sterilized since the polymer dextran remains in solutioneven at elevated temperatures. The effectiveness of this solution incontrolling corneal hydration in the complete absence of epithelium wascompared to a hypertonic salt solution containing sodium chloride at 5%concentration level in the following experiment:

The corneal epithelium was removed by chemical and mechanical means fromboth eyes of 15 rabbits. The solution was topically applied to the eyesat every two hours seven times a day. One eye received the dextransolution, the other eye was treated with the 5% sodium chloridesolution. The thickness of the cornea in each eye was determined bypachometry prior to the removal of the epithelium and once every dayduring the healing period. Pachometry is an optical method utilizing aslit beam of light that is reflected from the anterior and posteriorsurface of the cornea and is capable of determing corneal thickness tothe accuracy of ±0.01 mm. The average corneal thickness for each groupduring the first six days of the study is shown in Table I.

    ______________________________________                                                  Average corneal thickness ± S.D. in mm.                          Healing time (days)                                                                       Dextran solution                                                                            Hypertonic salt sol'n.                              ______________________________________                                        0           0.42 ± .01 0.41 ± .01                                       1           0.53 ± .02 0.65 ± .01                                       2           0.58 ± .01 0.68 ± .02                                       3           0.61 ± .03 0.71 ± .04                                       4           0.60 ± .02 0.72 ± .02                                       5           0.52 ± .03 0.72 ± .03                                       ______________________________________                                    

Purposely, the epithelial removal was extensive involving the limbus sothat no complete healing occurred during the first 5 days of treatment.

EXAMPLE II

The following composition was tested against a commercial hypertonicsolution containing 5% sodium chloride in a polymer base (Adsorbonac-5%,sold by Burton, Parsons, and Co., of Washington, D.C.):

    ______________________________________                                        Dextran-70           9.00   grams                                             Sodium chloride      0.90   grams                                             Benzalkonium chloride                                                                              0.004  grams                                             Disodium edetate     0.01   grams                                             Distilled water      90.10  milliliters                                       ______________________________________                                    

Forty rabbit eyes were deepithelialized but less extensively than inExample I. Half of the eyes were treated with the dextran solution, theother half were treated with the commercial hypertonic solution seventimes daily. The corneal thickness was determined daily and after 5days, the number of eyes completely healed were determined. The resultsare shown in Table II:

    ______________________________________                                                  Average corneal thickness ± S.D. in mm.                          Days of Treatment                                                                         Dextran solution                                                                            Hypertonic salt sol'n                               ______________________________________                                        0           0.40 ± .01 0.41 ± .01                                       1           0.62 ± .02 0.73 ± .02                                       2           0.63 ± .01 0.74 ± .01                                       3           0.60 ± .01 0.72 ± .01                                       4           0.58 ± .03 0.73 ± .01                                       5           0.55 ± .04 0.71 ± .03                                       No. of eyes healed:                                                                       10 (50%)      5 (25%)                                             ______________________________________                                    

EXAMPLE III

Forty rabbit eyes were burned with ultraviolet radiation on day 0 and onday 2. Ten eyes were treated with the following solution:

    ______________________________________                                        Dextran-40          6.00   grams                                              Sodium chloride     0.90   grams                                              Distilled water     93.1   milliliters                                        ______________________________________                                    

Another ten eyes were treated with the following solution:

    ______________________________________                                        Poly(vinyl pyrrolidone) M.W. = 40,000 daltons                                                          6.00   grams                                         Sodium chloride          0.90   grams                                         Distilled water          93.1   milliliters                                   ______________________________________                                    

Ten eyes were treated with 0.90% sodium chloride and other ten eyes weretreated with 5% sodium chloride solution at every 2 hours seven times aday. Corneal thickness was measured daily and the condition of theepithelium was graded on a subjective scale from 0 to 4+ after stainingwith Rose Bengal stain. The results are shown in Table III:

    ______________________________________                                        Treatment                                                                             Average corneal thickness (mm)/grade of epithelium                    Days    Dextran   Polyvinylpyrr                                                                             0.9% NaCl                                                                             5% NaCl                                 ______________________________________                                        0       0.40/0.0  0.41/0.0    0.39/0.0                                                                              0.40/0.0                                1       0.48/1.2  0.56/1.5    0.59/2.1                                                                              0.60/2.5                                2       0.52/0.9  0.60/2.0    0.61/2.0                                                                              0.62/2.3                                3       0.54/1.4  0.62/2.3    0.66/2.7                                                                              0.64/3.3                                4       0.50/1.0  0.61/2.1    0.64/2.5                                                                              0.65/3.0                                5       0.47/0.5  0.58/1.6    0.63/2.0                                                                              0.64/2.8                                ______________________________________                                    

EXAMPLE IV

6% Dextran-40 solution in isotonic saline preserved with benzalkoniumchloride solution in the presence of disodium edetate was used on twentypatients undergoing tonography. In this procedure, a cylindrical solidis pressed against the cornea, which is topically anesthetized, forseven minutes. Damage to the epithelium always occurs partially due tothe application of the anesthetic and partially from the mechanicaltrauma resulting from the prolonged contact of the cornea with a solidforeign body. Punctate staining of the epithelium with fluorescein andmoderate corneal edema are routinely observed in patients undergoingtonometry and these effects can last up to 48 hours. The dextransolution was applied to various groups of patients;

/A/ prior to the application of the anesthetic

/B/ after anesthesia but prior to tonography

/C/ after tonography

The cornea was stained with fluorescein immediately (I) and two hours(II) after tonography. At this time the degree of stromal haziness, anindication of the degree of edema, was also observed. The results areshown in Table IV:

    ______________________________________                                        FLUORESCEIN STAINING   STROMAL HAZE                                           GROUP   I.          II.        II.                                            ______________________________________                                        A       negligible  absent     absent                                         B       moderate    negligible absent                                         C       considerable                                                                              moderate   absent                                         Control*                                                                              considerable                                                                              considerable                                                                             present                                        ______________________________________                                         *isotonic saline was used in place of dextran solution.                  

EXAMPLE V

10% Dextran-70 solution in isotonic saline was used to irrigate thepreocular surface of patients undergoing intraocular surgery such asvitretomy or intraocular lens implant. The same solution but containingpreservatives was used topically seven times daily as postoperativetreatment on patients for two weeks following surgery. A significantreduction in corneal complications for the patients treated with thissolution was observed when compared to those receiving only conventionalmanagement.

EXAMPLE VI

Polyacrylamide polymer having an average molecular weight of about40,000 daltons and sodium chloride were dissolved in distilled water inthe following proportions:

    ______________________________________                                        Polyacrylamide      6.00   grams                                              Sodium chloride     0.90   grams                                              Distilled water     93.1   milliliters                                        ______________________________________                                    

This solution was tested in accordance with the procedure of Example I,above, with favorable results. Solutions prepared from polyacrylamidewere also employed in treating corneal edema and epithelialdiscontinuities in patients as well as a postoperative treatment aftersurgery involving the cornea both in Mexico and in Europe with promisingresults.

Other embodiments of the invention will be apparent to those skilled inthe art from a consideration of this specification or practice of theinvention disclosed herein. It is intended that the specification andexamples be considered as exemplary only, with the true scope and spiritof the invention being indicated by the following claims.

What is claimed is:
 1. A method of corneal dehydration in a human oranimal eye comprising applying thereto an effective cornea dehydratingamount of an aqueous solution of a hydrophilic nonviscous polyacrylamidehaving an average molecular weight of between about 40,000 and about150,000, said polyacrylamide being present at a colloidal osmolalitybetween about 40 and 200 torr and in a concentration of from about 5 to25% by weight.
 2. The method of claim 1, in which said ophthalmicsolution comprises, in addition, an eye compatible preservative.
 3. Themethod of claim 2 in which said preservative is chlorhexidinedigluconate.
 4. The method of claim 1 in which said ophthalmic solutioncomprises, in addition, an eye compatible anesthetic.
 5. The method ofclaim 4 in which said anesthetic is proparacaine.